Oil recovery process

ABSTRACT

An on-site, in-line process and system for recovering oil from oil-bearing subterranean formations which involves the production, modification, dilution and injection of a polymer solution, preferably consisting essentially of an aqueous solution of a partially hydrolyzed polyacrylamide, having injectivity and mobility properties capable of meeting the specific permeability requirements of substantially any subterranean formation to be achieved. The polymer solutions prepared by the process and system can be used as drive fluids for displacing oil (secondary polymer flood) in an oil-bearing formation, as mobility buffers to follow micellar dispersion floods in the conjoint presence of chemical reagents in other chemical floods (e.g., surfactant, caustic, etc.), or they can follow a water flood. The solutions can also be used to promote pipelining of high viscosity crude oil. Irrespective of the use to which the solutions are put, the process and system enable the polymer solutions to be customized, or tailor-made, so to speak, to meet the performance demands of the environment in which they are to be used, whether it be an oil-bearing formation or a pipeline.

TECHNICAL FIELD

The present invention relates to a process and system for recovering oilfrom oil-bearing subterranean formations or reservoirs, and, inparticular, to a process and system wherein a drive fluid, or mobilitybuffer, preferably in the form of an aqueous solution of a polymer suchas a partially hydrolyzed polyacrylamide is employed in the secondaryand tertiary recovery of oil from such formations or reservoirs.

BACKGROUND OF PRIOR ART

The recovery of residual oil from oil-bearing subterranean formations byflooding of the formation with an aqueous medium containing a polymersuch as a partially hydrolyzed polyacrylamide has received widespreadattention as evidenced by the substantial number of U.S. patentsdirected to the preparation and use of such solutions. Included amongthis large group of patents are U.S. Pat. Nos. 2,827,964, 3,002,960,3,039,529, 3,370,649, 3,558,759, 3,800,877, 3,825,067, 3,841,401, and3,853,802. At least one of the aforementioned patents, namely, U.S. Pat.No. 3,370,649, suggests the preparation and injection of an aqueoussolution of a partially hydrolyzed polyacrylamide at the site of theoil-bearing formation. In accordance with the teaching of that patent,polymerization and hydrolysis are carried out simultaneously in thepresence of an alkali metal polyphosphate and a suitable catalyst for atime sufficient to effect hydrolysis of from 5 to about 70 percent ofthe amide groups of the polymer. The resulting aqueous solution of thepartially hydrolyzed polyacrylamide is then mixed with water, andinjected into an input well penetrating a subterranean oil-bearingformation, and forced through the formation in the direction of one ormore output wells also penetrating the formation. The random, haphazard,all-in-one-pot production approach disclosed in the patent not only isinefficient and wasteful, and, therefore, economically unfeasible, but,also, and perhaps more importantly, yields an end product havingproperties which are unpredictable, and which, except by fortuitoushappenstance, are incapable of meeting the specific permeabilityrequirements of a subterranean oil-bearing formation of interest. Morespecifically in this latter connection, the process of U.S. Pat. No.3,370,649 does not enable the controlled production of an aqueoussolution of a polymer for use in an oil-bearing formation having thenecessary injectivity and mobility properties both at the input well toprevent, or substantially reduce, wellbore plugging, and, away from theinput well, that is, the matrix, to enable adequate displacement of oilin the formation in the direction of an output well to take place. Nor,moreover, does U.S. Pat. No. 3,370,649, or any of the otheraforementioned U.S. patents for that matter, suggest a system or processfor accomplishing such a result.

BRIEF SUMMARY OF INVENTION

In accordance with the present invention, a process and system have beenevolved for the production of aqueous polymer solutions for use inrecovering oil from subterranean oil-bearing formations which arecapable of meeting the performance demands of substantially anysubterranean oil-bearing formation. The process and system not onlyenable the production of polymer solutions having uniform andpredictable properties, but, also, enable the production of polymersolutions having superior properties, especially from the standpoint oftheir improved injectivity and mobility, their stability, their brinetolerance, and their resistance to degradation or thinning by shearforces. What is more, these results are achieved with greater efficiencyat a lesser cost than is possible with commercially available polymerproducts whether they are sold in solid form or in the form of aqueoussolutions.

The process and system of the present invention are adapted for on-site,in-line, essentially continuous use thereby eliminating the need forpurifying and storing the finished polymer solutions. To this end, theapparatus comprising the system advantageously may be preassembled andmounted on skids, for example, for ready transport to and from a sitewhere it is to be used. In its preferred embodiment, the system includesmonomer supply means, means for connecting the system to a source ofwater, polymerization means, catalyst feed and monitoring means,hydrolyzation means including means for feeding a controlled amount of ahydrolyzing agent into the polymer stream, and means for diluting thehydrolyzed polymer and injecting it into an input well penetrating areservoir of interest. The system further desirably includes watertreatment means, and means for introducing reaction control agents suchas oxygen, heat, inert gas, and polymerization accelerators into thesystem. The capability of the process and system for customizing, ortailor-making, a polymer solution which can meet the performance demandsof a reservoir of interest is centered upon making a determination ofthe average molecular weight and the molecular weight distribution of apolymer produced by the process and system. These measurements can bequantitatively correlated with the injectivity characteristics, themobility behavior, and the overall properties of the polymer. As aresult, a broad spectrum of polymers of varying molecular weights can beprepared to meet the permeability demands of substantially anyoil-bearing formation being worked. An important adjunct of the averagemolecular weight and molecular weight distribution measurements is aunique disc flooding technique which enables the injectivity andmobility properties of a polymer needed to achieve optimum displacementof oil from a formation to be predetermined. Once this determination hasbeen made, the parameters of the process and system can be changed toproduce a polymer having the desired properties.

As stated, the process and system are especially useful for thedisplacement recovery of petroleum from oil-bearing formations. Suchrecovery encompasses methods in which the oil is removed from anoil-bearing formation through the action of a displacement fluid or agas. Thus, the recovery may be secondary, where the reservoirhydrocarbons have been substantially depleted by primary recoverymechanisms, or it may be tertiary, where the polymer solution isinjected after injection of conventionally used displacement fluids.Other uses for the polymer solutions prepared by the process and systemof the invention include near wellbore injection treatments, andinjection along interiors of pipelines to promote pipelining of highviscosity crude oil. The solutions can also be used as hydraulicfracture fluid additives, fluid diversion chemicals, and losscirculation additives, to mention a few.

The foregoing, and other features and advantages of the invention willbecome clear from the description to follow, taken in conjunction withthe accompanying drawings, wherein:

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic representation of an embodiment of a system forpreparing polymer based drive fluids by the process of the presentinvention;

FIG. 2 is a graphical representation showing the linear relationshipbetween the molecular weight of a polymer prepared by the process ofthis invention and polymerization catalyst concentration;

FIG. 3 is a graphical representation showing the linear relationshipbetween the reciprocal relative mobility and the average molecularweight of a polymer prepared in accordance with this invention;

FIG. 4 is a graphical comparative representation of the molecular weightdistribution of a polymer produced by the process of this invention anda commercial product;

FIG. 5 is a schematic representation of a radial disc core obtained froman oil-bearing formation for use in determining the reciprocal relativemobility of a polymer of this invention at and away from the input well;

FIG. 6 is a graphical representation of the mobility of a polymersolution prepared by the process of this invention as a function ofinjection volume;

FIG. 7 is a graphical representation of the mobility of polymer solutionprepared from a typical commercial polymer as a function of injectionvolume; and

FIG. 8 is a comparative graphical representation showing the effect ofsalinity on the mobility of a polymer solution prepared by the processof this invention and on a commercial product.

DETAILED DESCRIPTION OF INVENTION

While the process and system of the present invention can be adapted forthe production of various polymers for use in preparing aqueoussolutions to be employed as drive fluids, or mobility buffers, in therecovery of oil from oil-bearing subterranean formations, in accordancewith the preferred practice of the invention, the polymers are preparedfrom a water soluble monomer containing at least one vinyl groupingwherein the vinyl group is an acrylyl vinyl, a vinyl cyanide, a styrylvinyl, or a water soluble salt thereof. When the vinyl grouping is anacrylyl vinyl, the monomer may be represented by the formula:

    CH.sub.2 ═CY--CO--X

wherein X is hydrogen, an amino group (NH₂), hydroxy, methyl or an ORgroup, wherein R is a lower alkyl radical, and wherein Y is hydrogen ora methyl group. Exemplary of monomers having utility in the practice ofthe invention are acrylamide, acrylic acid, acrylonitrile, methacrylicacid, methacrylamide, methacrylonitrile, methyl methacrylate and sodiumstyrenesulfonate. Of this group, acrylamide is the preferred monomer.The acrylamide can be any of various commercially availablepolymerization grade acrylamides sold in solid form, or in the form ofaqueous solutions. From the standpoint of safety considerations, theaqueous solutions are preferred because they eliminate the dust problemswhich characterize the solid form of the monomer.

The polymerization of the monomer is carried out in the presence of asuitable vinyl polymerization initiator or catalyst. Especially usefulare free radical forming compounds such as the azo compounds exemplifiedby azobisisobutyranitrile and azobisisobutyamidine chloride; peroxidessuch as hydrogen peroxide, sodium peroxide and benzoyl peroxide; alkyland dialkyl peroxides such as, for example, t-butyl hydrogen peroxidesand diethyl peroxide; alkali metal, including ammonium, persulfates suchas sodium persulfate, potassium persulfate and ammonium persulfate; andalkali metal bisulfites exemplified by sodium bisulfite and potassiumbisulfite. Where the monomer to be polymerized is acrylamide, ammoniumpersulfate and sodium bisulfite, especially co-mixtures thereof employedin a ratio of about 9:1 to about 1:9, are preferred as the catalysts orfree radical initiators. The rate of polymerization of acrylamidemonomer using a co-mixture of ammonium persulfate and sodium bisulfitecan be accelerated by replacing a portion of the sodium bisulfite with aferrous compound such as ferrous ammonium sulfate. Excellent results areobtained in a tricatalyst system of this type where from about 20 toabout 40, preferably about 30 to about 35, mole percent of the sodiumbisulfite is replaced by the ferrous compound. As shall be discussed ingreater detail below, the concentration of the free radical initiator orcatalyst used in the polymerization step of the process of thisinvention plays an important role in providing an aqueous polymersolution capable of meeting the performance demands of a specificoil-bearing subterranean formation.

In accordance with one aspect of the invention, the performancecapabilities of acrylamide polymers produced by the process have beenenhanced by carrying out the polymerization in the presence of certainorganic polycarboxylic acids exemplified by ethylenediaminetetraaceticacid, N-(2 hydroxyethyl)-ethylene-diaminetetraacetic acid, andnitrilotriacetic acid. Thus, by way of illustration, in thepolymerization of acrylamide, the addition of from about 250 to about350 ppm of ethylenediaminetetraacetic acid to the reaction mixtureprovided a polymer having superior performance in core floods.

It is noteworthy that the process of the present invention utilizesoxygen to initially inhibit polymerization of the monomer and, yet,enables the polymerization to be carried out in the presence of oxygen,although in a concentration less than that when it is used as aninhibitor. The oxygen can be pure oxygen. Preferably, however, air isemployed as the oxygen source. The oxygen desirably is introduced intothe reaction mixture by bubbling it into the mixture until saturation isreached. At ambient temperatures, in a reaction mixture comprising awater solution of acrylamide wherein the concentration of the acrylamideis of the order of from about 1 to about 8 weight percent, oxygensaturation is reached at about 3 to about 10 ppm, usually about 4 toabout 6 ppm by weight of oxygen. At this concentration, the oxygen actsas a polymerization inhibitor. After the concentration of the oxygen hasbeen reduced to about 0.1 to about 0.2 ppm, as by sparging with an inertgas, vacuum degassing, or by introducing an oxygen scavenger such assodium bisulfite, or a combination thereof, polymerization of themonomer proceeds.

In general, it is preferred to use tap water in the process. Theadvantage in using tap water is that it has a low salt level, and the pHof the water is in the range of 8-10 which favors the polymerizationreaction. While tap water is preferred, it is possible to use a range ofwaters from deionized to connate (natural reservoir water).

The temperature at which the polymerization is conducted is somewhatvariable. In the polymerization of acrylamide, for example, thetemperature of the reaction mixture desirably should be in the range offrom about 30° C. to about 80° C., preferably from about 40° C. to about55° C. Boiling should be avoided. Polymerization times, likewise, arevariable. Again, using acrylamide as the monomer, polymerization of themonomer is permitted to proceed for a period of from 8-12, usually about9-10 hours. The polymerization reaction is exothermic, and the reactionmixture, after the initial reaction period, is stirred at the terminalexotherm temperature of the mixture for an added 1 to 4, generally 2 to3, hours.

When polymerization is completed, an amount of a monovalent base such assodium or potassium hydroxide is added to the polymer to hydrolyze fromabout 20 to about 40 mole percent of the amide groups in the case of apolymer such as polyacrylamide, or ester groups, in the case of apolymer such as polymethylmethacrylate. Where the polymer ispolyacrylamide, the generally optimum objectives of the invention fromthe standpoint of injectivity and mobility control, and overallperformance capabilities, are attained with a partially hydrolyzedpolyacrylamide in which from about 25 to about 35, especially desirablyabout 30 mole percent, of the amide groups have been converted tocarboxylate groups.

As indicated, one of the important aspects of this invention is theability to provide, at the point of use, an aqueous polymer solutioncustomized, or tailor-made, to meet the demands of any particularoil-bearing subterranean formation. To this end, a system, thecomponents of which may be mounted in a manner to enable them to beeasily transported to a preselected site, has been evolved. Anembodiment of such a system is schematically represented in FIG. 1 ofthe drawings. As shown, the system essentially comprises monomer supplymeans 10 and a source of water 12 in communication with a polymerizationreactor or vessel 16. A catalyst source 18 also advantageously is incommunication with the vessel 16. The catalyst source 18 desirablycomprises means for holding, separately, each component of a cocatalyst,and means for metering a predetermined amount of each component of thecocatalyst separately, or together, into the vessel 16. The catalystsource 18 and its associated metering means (not shown) are importantfeatures of the system in that through proper adjustment of theconcentration of the catalyst, or cocatalyst, in the vessel 16 theaverage molecular weight and the molecular weight distribution of thepolymer formed in the vessel 16 can be altered as desired to provide apolymer having the injectivity and mobility properties capable ofmeeting the performance demands of a reservoir of interest. The vessel16 desirably has a stirrer or mixer 16a, and is provided with a steamjacket 20 for heating the reaction mixture in the vessel. The vessel 16has provision for introducing nitrogen or other inert gas into thevessel 16 for removing, or substantially reducing, the concentration ofoxygen in the reaction mixture. A pump 22, having its inlet end incommunication with the vessel 16, and its outlet end in communicationwith a mixing valve 24 is provided for removing the formed polymer fromthe vessel 16 and into contact with an aqueous base solution from asource 26 thereof also in communication with the valve 24. The mixingvalve 24 is connected by a line to mixing means which, in the embodimentof the system illustrated, comprises static mixers 28. The static mixers28, as shown, in turn, are in communication with a hydrolysis reactor orvessel 30 where dilution of the formed polymer occurs. A pump 32 isconnected to the vessel 30 for removing the diluted polymer andtransferring it to a final dilution mixer or unit 34. The dilutedpolymer solution formed in the unit 34 is ready for injection into aninput well. The system desirably may incorporate auxiliary equipment(not shown) including a monomer chelating and weighing tank, a source ofa polymerization accelerator such as ethylenediamineacetic acid, waterdemineralizing means, ion exchange means for removal of Cu⁺⁺ from themonomer solution, and a scrubber for the water introduced at the finalpolymer dilution stage of the system.

By way of illustration, a monomer such as acrylamide, which may be inthe form of a solid, or a commercially available 50%, by weight, Cu⁺⁺inhibited aqueous solution, is introduced into the vessel 16 along withwater from the source 12. If the monomer is in the form of an aqueoussolution of the type mentioned, the solution advantageously is firstpassed through an ion exchange resin to remove Cu⁺⁺. The aqueous monomersolution in the vessel 16 will comprise about 6%, by weight, monomer andwill have a pH in the range of from about 8 to about 10. Theconcentration of oxygen in the solution will be of the order of about 4to 6 ppm. The monomer solution is heated to a temperature of about 40°C. to about 45° C. by means of the steam jacket 20. The heated monomersolution is then sparged with nitrogen until the oxygen concentration isreduced to a level of about 0.1 to about 0.2 ppm. Sparging rates aresomewhat variable. Generally speaking, they should range from about 0.1to 10, preferably 1 to 2 volumes of gas per volume of solution. Assumingfor purposes of this illustration that a cocatalyst comprising sodiumbisulfite and ammonium persulfate is employed, sodium bisulfite from thesource 18 is first introduced into the vessel. The bisulfite acts as anoxygen scavenger, and augments the deoxygenation achieved with thenitrogen gas. Ammonium persulfate from the source 18 is then added tothe reaction mixture. The concentration of the components of thecocatalyst will be about 180 ppm of the bisulfite and about 400 ppm ofthe persulfate, based upon the weight of the acrylamide monomer. Theconcentration, of course, can be varied as pointed out above to providea polymer having the desired average molecular weight and molecularweight distribution. The relationship of catalyst to the molecularweight of the polymer is graphically depicted in FIG. 2 of the drawings.As shown, the average molecular weight of a polymer is a linear functionof the inverse square root of the concentration of the catalyst. Thus,by selecting the appropriate catalyst level, it is possible with theprocess and system of this invention to prepare at the point of use apolymer having a desired average molecular weight, and one which, asstated, will be capable of meeting the performance demands of thereservoir. Following addition of the cocatalyst, the mixture in thevessel 16 is allowed to react for from 8 to 12 hours. The reactionexotherms until a temperature of from about 55° C. to about 60° C. isreached. The reaction mixture is stirred for approximately 2 to 3 hourswhile at this temperature. The 6% polymerized solution is thereafterpumped from the vessel 16 and mixed with a 50% solution of sodiumhydroxide from the source 26. The rate of feed of the aqueous caustic issuch that about 0.3 mole of the hydroxide is admixed with about 1 moleof the polyacrylamide, that is, sufficient caustic is introduced tohydrolyze approximately 30% of the amide groups comprising the polymer.From the mixing valve 24, the polymer solution is conveyed to the staticmixers 28. No thinning or degradation of the polymer due to shear forcesoccurs in mixers 28, or in the subsequent dilutions of the polymer. Fromthe mixers 28, the partially hydrolyzed polymer solution is tranferredto the vessel 30 where it is mixed with fresh water from the source 12to form about a 1%, by weight, aqueous solution of the partiallyhydrolyzed polyacrylamide. The solution remains in the vessel 30 forabout 10 to about 12 hours, and is then transferred by means of the pump32 to the dilution mixer 34 to provide an aqueous solution containingabout 50 to about 5,000 parts per million, of partially hydrolyzedpolyacrylamide. From the mixer 34, the diluted solution can be injecteddirectly into an input well, or it can be transferred temporarily to astorage holding tank where it will be ready for injection into an inputwell.

The process and system of this invention, as mentioned above, enablesthe on-site preparation of polymers having a selectively controllablespectrum of molecular weights whereby a polymer can be customized, ortailor-made, to meet the permeability demands of substantially anyreservoir of interest. More specifically, the process and system of thepresent invention enables the preparation of polymers, especiallypolymers such as partially hydrolyzed polyacrylamide, having aselectively controllable average molecular weight and a molecular weightdistribution such that a quantitative correlation can be made betweenthose parameters and the injectivity and mobility behavior of thepolymer in a reservoir. The relationship of these parameters, that is,average molecular weight and reciprocal relative mobility (RRM) isgraphically depicted in FIG. 3 of the drawings. The RRM of a partiallyhydrolyzed polyacrylamide is shown as a linear function of the averagemolecular weight, and this relationship provides a valuable tool incustomizing, or tailor-making, a polymer to meet the performance demandsof a formation. A unique adjunct to the process and system is the use ofa disc flooding technique which enables the reciprocal relativemobilities not only of the polymers prepared by the process of thisinvention, but also, the mobilities of drive fluids such as micellardispersions, to be ascertained with statistically significant accuracyat substantially any given distance from the injection site. Thisinformation provides a high degree of predictability of the performanceof the polymer and/or fluid in a particular reservoir, and results inoptimum oil recovery.

By way of background, it is known that the reduction of the mobility ofa fluid in a porous media such as an oil-bearing reservoir can beaccomplished by increasing the viscosity of the fluid, decreasing thepermeability of the porous media, or by a combination of both. Partiallyhydrolyzed polyacrylamides both increase the viscosity of water andreduce the permeability of a reservoir as their solutions flow throughit. The extent to which a particular concentration of a given partiallyhydrolyzed polyacrylamide performs these two functions is very roughly afunction of the polymer's average molecular weight. The viscosity, atlow shear, and the screen factor of the polymer are simply bench-toptechniques for comparing average molecular weights and are routinelyused for quality control. The screen factor measurement is related tothe permeability reduction capabilities of the partially hydrolyzedpolyacrylamide.

A sample of a polymer having a higher molecular weight will give ahigher viscosity and screen factor than a lower average molecular weightsample at the same concentration. Conversely, a higher molecular weightsample will require lower concentrations to provide the same viscosityand screen factor. It is reasonable to conclude from this that thehigher molecular weight polymers should be more efficient. However, thisis not the case in many instances. Thus, for example, where extremelylarge partially hydrolyzed polyacrylamide molecules are present, theymay visually appear to go into solution but actually form gel-likesystems which act as discrete particles. These particles are capable of,and do, in fact, filter out on the sand face or are entrapped in thefirst few centimeters of subsurface material and act to "plug" thewellbore. As a result, they substantially reduce injectivity of thepolymer solution without contributing to mobility reduction further intothe reservoir.

Generally speaking, the lower the permeability of the reservoir, thelower is the average molecular weight of the polymer which can beinjected without significant wellbore plugging. For a given formation,however, it is entirely possible to have two partially hydrolyzedpolyacrylamide solutions of the same average molecular weight which willexhibit radically different efficiencies for mobility control purposes.Where the molecular weight distribution of a polymer is relativelynarrow, as is the case with the polymers produced in accordance with theprocess of this invention, substantially all of the polymer is effectivein injectivity and mobility control. On the other hand, where themolecular weight distribution is broad, as in the case of most presentlycommercially available partially hydrolyzed polyacrylamides, themobility is adversely affected by the lower molecular weight moleculesin the polymer mixture, while the higher molecular weight molecules ofthe polymer indicate the presence of gel-like species that result inwellbore plugging.

It is reasonable to conclude from the foregoing discussion that the mostsignificant polymer property is molecular weight. Polymers, especiallyman-made polymers, are complex mixtures of molecules of varyingmolecular weights. For this reason, the molecular weights of polymersare measured and reported as average molecular weights (M). In theory, anumber of methods are available for determining the average molecularweight of a polymer. Included among these methods are light scattering,gel permeation chromatography, electronmicroscopy, andultracentrifugation, to name a few. In accordance with the preferredpractice of this invention, ultracentrifugation is employed to obtainthe average molecular weight characteristics of polymers produced by theprocess and system described above. In addition to enabling themeasurement of the average molecular weight of a polymer,ultracentrifugation provides information which enables the calculationof the molecular weight distribution of the polymer. This is asignificant measurement in that it provides information on the weightedrange of the diverse molecular weight species comprising the polymer. Anaccurate knowledge of the average molecular weight and molecular weightdistribution of a polymer species allows one to correlate not only theperformance of a polymer in a porous media, but, also, polymer solutionproperties such as screen factor and Brookfield viscosity, with afundamental and important polymer property, namely, molecular weight.

Basically, ultracentrifugation is a technique for measuringsedimentation rates. Analytical ultracentrifuges capable of providingsuch data are commercially available, excellent results being attainablewith a Beckman Model E Analytical Ultracentrifuge. On the basis that alarge mass (high molecular weight) sediments faster than a small mass(low molecular weight), as measured at the high angular velocities atwhich the ultracentrifuge operates, the average molecular weight (M) ofa polymer can be calculated from the sedimentation data provided by theultracentrifuge utilizing a combination of the Svedberg equation (1),and the Flory-Mendelkern-Schrage equation (2). The equations are shownbelow: ##EQU1## where: S_(o) is the sedimentation coefficient atinfinite dilution

D_(o) is the diffusion coefficient at infinite dilution

K is the gas constant

t is temperature

v is the polymer partial specific volume

ρ_(o) is the solvent density ##EQU2## where: S_(o) is the sedimentationcoefficient at infinite dilution

[μ] is the intrinsic viscosity

μ_(o) is the solvent viscosity in poise

ρ_(o) is the solvent density

N is Avogadro's number

β is a constant related to the polymer frictional coefficient

v is the polymer partial specific volume

The quantities S_(o) and D_(o) in equation (1) are obtained by plottingthe sedimentation and diffusion coefficient data obtained from theultracentrifuge against the concentration of the polymer underinvestigation, and then extrapolating the essentially linearrelationship of these parameters to infinite dilution to get S_(o) andD_(o). The parameter v, that is, the polymer partial specific volume,cannot be evaluated experimentally by the usual pycnometric methodbecause viscosity precludes preparation of sufficiently concentratedsolutions. Therefore, the value of v is determined using a sophisticateddensitometer (Mettler DMA 55) which allows the measurement of highviscosity solutions. The β constant for numerous polymers other thanpartially hydrolyzed polyacrylamides are available. A suitable model waschosen from these to get a close approximation of this parameter. Singlestrand DNA provides an excellent model since it is of comparablemolecular weight, is linear, and has pendant charged groups. The βconstant for DNA is 2.51×10⁶, and this is the value used in making thecalculations from the data obtained from the ultracentrifuge.

In addition to providing data for measuring the average molecular weightof a polymer species, ultracentrifugation provides information for thecalculation of molecular weight distribution of a polymer species. Asstated, this is a highly useful measurement in that it providesimportant knowledge with regard to the weighted range of the variedmolecular weight characteristics of a polymer species. For any givenpolymer species, the optics of the ultracentrifuge generate a pattern,which is registered refractometrically, representing the concentrationgradient of the polymer versus the distance from the center of rotation.The gradient curves thusly generated are transformed directly into adistribution of sedimentation coefficients which can be converted to adistribution in molecular weight. A quantitative measure of themolecular weight distribution is obtained by taking the moments of thedistribution curves generated by the optics of the ultracentrifuge anddetermining the standard deviation of molecular weight of abundance fora particular molecular species present in a selected polymer sample.This value is represented by the symbol σ (sigma). Since the molecularweight distribution is dependent upon the average molecular weight ofthe polymer, the relative width or spread of the molecular weightdistribution of the polymer is determined by dividing the value obtainedfor σ from the distribution curves by the average molecular weight (M)of the polymer.

The foregoing calculations can be made for a large number of polymerspecies prepared by the process of this invention to provide ameaningful correlation between the average molecular weight of a polymerand its mobility. This relationship is shown graphically in FIG. 3 ofthe drawings to which reference has been made above.

The generally optimum objectives of the present invention are met withpartially hydrolyzed polyacrylamides having an average molecular weightin the range of from about 2 to about 10 million, preferably from about4 to about 7 million, and a molecular weight distribution, as obtainedby the relationship σ/M, of from about 0.02 to about 0.22. Thereciprocal relative mobility (RRM) of the polymer at the aforesaidaverage molecular weight range will be in the range of from about 1 cpto about 1000 cp.

As stated previously hereinabove, an important aspect of providing apolymer solution of predictable properties which is customized, ortailor-made, to meet the specific demands of a reservoir of interest, isthe unique core or disc flooding technique of the present invention. Thetechnique enables the mobility behavior of a polymer, both at thewellbore and in the matrix of a formation, to be predicted with anappreciable degree of accuracy. The information thus provided can beused to alter the processing parameters employed in the system asdescribed above to provide a partially hydrolyzed polyacrylamidesolution capable of meeting the demands of a formation at the point ofuse to enable optimum recovery of oil to be attained.

The data obtained from the core or disc flooding technique of theinvention to achieve optimization of polymer flooding includes (1) oilrecovery, (2) mobility behavior, and (3) polymer retention. Oil recoveryis measured as a function of injection volume by incremental producedfluid sampling throughout the flooding operation. Mobility data iscalculated from pressure drop data continuously monitored over the spanof the flooding operation. Polymer retention is determined by materialbalance on injected and produced polymer in the aqueous phase.

The effectiveness of polymer flooding is determined with the core ordisc flooding technique by comparing oil recovery efficiency of thepolymer flood to that of water flooding. Polymer flooding may be appliedat any time in an oil reservoir's life, that is, immediately afterprimary production in lieu of water flooding (secondary polymer flood),or it can, and is more likely to, be applied later in a reservoir's lifeafter secondary water flooding (tertiary polymer flood). The core ordisc flooding technique of this invention is utilized to optimizepolymer flooding for both secondary and tertiary flooding practices.Incremental polymer flood oil recovery, that is, the volume of oilrecovered from the core or disc sample in excess of that recovered bywater flooding, is determined. In a tertiary flood, the core or disc is(1) restored to initial oil saturation by injection of crude oil fromthe reservoir, (2) water flooded (generally 2 PV of water is injected),and (3) polymer flooded (generally 2 PV of polymer solution plus drivewater is injected). Incremental polymer flood oil recovery is the amountobtained in Step 3. In a secondary flood the core or disc is (1)restored to initial oil saturation by crude oil injection, (2) waterflooded, (3) restored back to initial oil saturation by crude oilinjection, and (4) polymer flooded. Incremental polymer flood oilrecovery is that obtained in Step 4 less that obtained in Step 2.

Typically, incremental polymer flood oil recovery ranges from about 2%PV to about 15% PV. Incremental oil recovery preferably is at least 2%PV in order to be significant, and advantageously is 5% PV, or greater.The actual volume (measured in milliliters) of incremental oil producedis related to the disc pore volume. The pore volume of a particular coreor disc is dependent upon its dimensions (height and diameter) andporosity.

In carrying out the core or disc flooding technique, a core or discsample is taken from anywhere in the area of the reservoir of interest.The disc may be either oil wet and/or water wet representing a spectrumof oil saturation. The disc sample normally will measure about 5 toabout 6 inches in diameter and will be about 2 inches in height. Anapproximately 1/8 inch bore is made in the center of the disc forinjecting fluids. The disc is separated into a number of concentricrings by means of pressure taps located along a radius. The pressuretaps are in contact with the upper disc surface only. A schematicdiagram of a typical disc is shown in FIG. 5 of the drawings. The discis enclosed in a holder which, in sharp contrast to conventionaltop-to-bottom, or lengthwise, core test procedures, only permits flow offluids to take place laterally, or radially, from the center of the discto the outer wall of the disc, a condition which more closely emulatesthe flow characteristics of a fluid injected into a reservoir. Mobilitybehavior calculated from pressure drop data is continuously monitoredthroughout the span of the disc flood. Reciprocal relative mobility(RRM), is calculated from Darcy's equation (3) ##EQU3## is onlydependent upon pressure drop (Δp) since permeability (K), height (h),flowrate (Q) and inner (r₁) and outer (r_(o)) radii are constant foreach disc flood. Pressure drops are larger near the center wellbore dueto larger r_(o) /r₁ ratios. Absolute values of pressure drops duringpolymer injection are dependent upon the K constant in Equation 3 andeffective viscosities times permeability reduction.

The optimum configuration for a polymer is determined by conducting discfloods wherein both the concentration and volume of polymer injected arevaried and polymer flood incremental oil is measured. The optimum dosage(concentration times pore volume) is that which gives near maximumincremental oil recovery (i.e., where increased polymer dosages givevery little additional incremental oil recovery). The optimumconfigurations of other polymers differing in molecular weight aredetermined in a similar manner. Lower concentrations are required toobtain maximum incremental oil recovery with increasing molecular weightpolymers.

Polymer mobility and retention data are useful in selecting the optimummolecular weight. Preferably polymer retention should be less than about200 lb/AF (Acre Foot), and most advantageously less than 100 lb/AF. Itis desirable that the maximum produced polymer concentration be greaterthan about 50% of the injected concentration to effect water mobilityreduction and incremental oil recovery at appreciable distances from theinjection or input wellbore. In instances where polymer retention isvery high, that is, above about 500 lb/AF, polymer flooding is probablyuneconomical. The subterranean rock composition is the dominant factorcontrolling polymer retention. Other factors affecting retention,although to a somewhat lesser degree, are polymer type and watersalinity, both of which can be controlled by the process and core ordisc flooding technique of this invention.

The mobility data obtained by the disc flooding technique is extremelyuseful in optimizing polymer molecular weight. Injectivity is also animportant economic factor. Injectivity is inversely proportional to thetotal reciprocal relative mobility of a polymer. Reciprocal relativemobility increases and injectivity decreases with increasing polymermolecular weight. Generally speaking, the following criteria areemployed in selecting a polymer of optimum molecular weight: (1) thereciprocal relative mobility of the polymer should be about 3 to about10 times greater than that of water, (2) the reciprocal relativemobility of the drive water following the polymer flood should be lessthan a factor of about 5 times greater than that of water prior topolymer injection, and (3) during polymer flow the reciprocal relativemobility in ring 1 of the disc should be lower than those in the outerdisc rings. Polymers with higher molecular weights which may affectmaximum incremental oil recovery at lower polymer concentrations are noteconomically optimum due to their reduced injectivity. The thirdmobility characteristic listed above is unique to partially hydrolyzedpolyacrylamides prepared in accordance with the teachings of thisinvention. Commercial polymers generally cause the highest reciprocalrelative mobilities in ring 1 of the disc due to excessive near wellboreplugging. This is extremely undesirable since total reciprocal relativemobility, and, hence, injectivity is dominated by the near wellborebehavior in a radial system. These fundamental differences in thepolymers of the present invention and commercial polymers will beexpanded upon later in the description.

By way of illustration, a disc sample was obtained from an oil-bearingformation in a Western reserve where water flooding had been carriedout. The disc had a radius of 6.20 cm and a height of 4.76 cm. A 1/8inch wellbore was drilled in the center of the disc, and the disc waspurged of residual fluids with a suitable solvent. Following drying, thedisc was enclosed in a holder having internal dimensions slightly largerthan the outer dimensions of the disc, and pressure taps were insertedin openings in the cover of the holder along a radius thereof to dividethe disc into four concentric rings as shown in FIG. 5. The dry disc wasthen saturated, by injection through the 1/8 inch wellbore, withformation water, to determine the permeability characteristics of thedisc using Equation 3. The core or disc pore volume was found to be146.2 cc, and the porosity of the disc was 25.4%. The residual oilsaturation of the disc was 68.4% PV. The water saturation was 31.6% PV.The disc was then saturated with crude oil from the reservoir from whichthe disc sample had been taken to duplicate as closely as possible inthe disc the natural conditions of the reservoir. The viscosity of thecrude oil was approximately 10 cp at 85° F. Following saturation withthe crude, approximately 2 PV of water having a viscosity of 0.82 cp at85° F. was injected into the disc until oil production ceased (100%water cut). The crude oil was collected through a tap in the disc holderat the outer edge or wall of the enclosed disc. The total volume of oilobtained was approximately 16.4 ml, or 11.22% PV. After the oil had beenflushed from the disc with the water flood, 0.5 PV of a polymer solutionprepared by the process of this invention was injected into the disc ata constant flow rate of 13 cc/hr. The concentration of the polymer inthe solution was approximately 1000 ppm, and the average molecularweight and the molecular weight distribution of the polymer were 4million and 0.15, respectively. The injected polymer solution was notscreened or filtered in any manner. The viscosity of the solution wasapproximately 7.84 cp at 85° F. The polymer flood was immediatelyfollowed by a water drive. The volume of oil recovered by the polymerflood was 13.35 ml or 9.13% PV. The amount of polymer injected was 71.80mg. The amount of polymer produced was 58.64 mg. The total polymerretained was 13.25 mg, giving a polymer loss due to retention of 63lb/AF.

The pressure drop across the disc was continuously monitored. Thus, forrings 1, 2 and 3 during the water flood, the pressure drop for ring 1ranged from an initial high of 1.32 psi to a low of 0.14 psi; for ring2, from 0.75 psi to 0.10 psi; and for ring 3, from 0.13 psi to 0.02 psi.The total of the pressure drop across all of the rings is the sum of0.14, 0.10 and 0.02 or 0.26 psi. During the polymer flood, the drop inpressure for ring 1 ranged from a high of 1.25 psi to a low of 0.74 psi;for ring 2, from 0.35 psi to 0.19 psi; and for ring 3, from 0.04 psi to0.08 psi. The total drop in pressure was 1.01 psi. The correspondingfigures for the polymer water drive were 0.14 psi to 0.05 psi for ring1; 0.10 psi to 0.05 psi for ring 2; and 0.06 psi to 0 for ring 3. Thetotal pressure drop for the water drive was 0.10 psi.

The reciprocal relative mobility of the polymer solution is equivalentto the viscosity term (λ_(r) ⁻¹) of Equation 3 using the initial waterpermeability of the disc as K in the equation. Since the averagemolecular weight of the injected polymer solution is known frompreviously obtained ultracentrifuge data, the reciprocal relativelymobility of the polymer in the reservoir can be predicted. More than onepolymer solution may have to be injected into the disc to determine thepolymer solution best suited to meet the demands of the reservoir.However, this determination can be made quickly, and, certainly, moreefficiently and far less expensively than conducting trial and errortests in the field. The overriding consideration is that the techniqueenables one to preselect a polymer solution capable of providing optimumperformance at minimal concentration of the polymer without anyconcomitant face plugging at the injection well.

The core or disc flooding technique of this invention can also be usedto advantage in the preselection of micellar flooding materials. Suchmaterials are the subject matter of a number of U.S. patents includingPat. Nos. 3,266,570, 3,506,070, 3,682,247 and 3,734,185. Generallyspeaking, these materials comprise a dispersion consisting essentiallyof water, hydrocarbon and surfactant. Optionally, an electrolyte and/orcosurfactant can be added. The dispersions are classified aswater-external or oil-external depending upon whether the hydrocarbonphase is internally dispersed or the water phase is internallydispersed. Also, the dispersions can be classified as intermediate wherethe external phase is not classically defined as either water-externalor oil-external. In the usual case, the injection of about 1% to about20% formation pore volume of a micellar dispersion provides efficientrecovery of crude oil from oil-bearing formations. The nature ofmicellar dispersions is such that, especially from an economicstandpoint, it is important to ascertain, before injection into areservoir, the performance characteristics of the dispersion in areservoir of interest. The core flooding procedure described above inrelation to the polymer solutions produced by the process of thisinvention can be utilized to advantage to customize or tailor-make, soto speak, a micellar dispersion to satisfy the demands of substantiallyany reservoir where the use of such dispersions are feasible. The coreflooding procedure determines the mobility profile of the dispersion.This property enables the dispersion to achieve optimum recovery of oilin the reservoirs. The viscosity can be readily changed for this purposeby adjusting the water content of the dispersion among other variables.

In using micellar dispersions in the secondary and tertiary recovery ofoil, it is a preferred practice to follow the micellar dispersion slugwith one or more mobility buffer slugs, the latter usually beingfollowed by a water drive. U.S. Pat. No. 3,406,754 discloses a processwhere this practice is employed. The mobility profile of the micellardispersions and the mobility buffers used in the process advantageouslyare graded from a low mobility equal to or less than the mobility of theformation fluids, that is, the crude oil and water within the formation,to a high mobility equal to or approaching that of the injected waterdrive. The mobility of the front, midportion, and back portion of themobility buffer, or buffers, and, optionally, the micellar dispersion,should be designed to be compatible with the formation fluids as well aseach preceding and following slug, including the water drive, used inthe process to attain optimum displacement of oil and to avoid adverseeffects on adjacent slugs due to fingering and/or leaching of thecomponents comprising each slug. The process and system of the presentinvention as described above not only enable the micellar dispersion andmobility buffers to be designed to meet the performance demands of areservoir of interest, but, also, provide for the on-site preparation ofmobility buffers having optimum injectivity and mobilitycharacteristics.

More specifically in this latter connection, the process and system ofthis invention enable the on-site preparation of mobility buffers ofdiffering molecular weights, and, as pointed out above, differingreciprocal relative mobilities, such that the front, midportion and backportion of each slug will have optimal internal compatibility as well asoptimal external compatibility with respect to the preceding andfollowing slugs, including the micellar dispersion and the water drive,used in the oil recovery operation. This is achieved in accordance withthe practice of this invention by selectively altering the concentrationof the catalyst, or cocatalyst, used in effecting polymerization of themonomer employed.

In order to demonstrate the superior performance properties of thepartially hydrolyzed polyacrylamide solutions prepared by the processand system of the present invention and a leading commercially availablepartially hydrolyzed polyacrylamide, namely, PUSHER 700 (Dow ChemicalCompany), comparative studies were carried out. FIG. 4 is a schematicgraphic representation of molecular weight distribution curves obtainedwith a partially hydrolyzed polyacrylamide prepared by the process ofthis invention, and a sample of PUSHER 700 polymer. The polymer preparedby the process of this invention has an appreciably narrower molecularweight distribution devoid of the extreme low and high molecular weightspecies of the commercial product. The narrower molecular weightdistribution of the polymer enables more efficient and uniform mobilitycontrol to be attained since substantially all of the polymer iseffective in contributing to the mobility properties of an aqueoussolution of the polymer. In the case of the PUSHER 700 polymer, themolecular weight distribution of the polymer is much broader, and thepolymer contains an appreciable proportion of extremely large partiallyhydrolyzed polyacrylamide molecules indicative of the presence ofgel-like species. Gel-like particles tend to filter out on the sandface, or are entrapped in the first few centimeters of sand, and act to"plug" the wellbore. Thus, they reduce injectivity without contributingto mobility control further into the matrix of a reservoir. Excessiveplugging in the near wellbore region results in wanton loss ofinjectivity.

The sharp differences in the injectivity and mobility properties of apartially hydrolyzed polyacrylamide of this invention and the PUSHER 700polymer product are depicted in FIGS. 6 and 7 of the drawings. The testresults were obtained using multitapped fired Berea discs with 2.5 porevolume injection of 1000 ppm polymer in 500 ppm sodium chloride brinefollowed by 2.5 pore volume brine at an injection rate of approximately0.0056 cc/sec. As shown in FIG. 6, the reciprocal relative mobility ofthe partially hydrolyzed polyacrylamide of the present inventionincreases with increasing radial distance. This is a result ofrelatively low and uniform permeability reduction including the nearwellbore region (ring 1) and a high viscosity which increases withdistance. As shown in FIG. 7, the mobility profile of the commercialproduct, PUSHER 700 polymer, is more complex. Initially reciprocalrelative mobility decreases with radial distance. In this zone,permeability reduction dominates viscosity. Permeability reduction isextremely high at the wellbore region (ring 1) and decreases withdistance, at which time it becomes constant at a comparatively lowvalue. After permeability reduction becomes constant, the effects ofrelatively low, but increasing viscosity causes the reciprocal relativemobility to increase with distance. This comparative study shows thatthe polymer of this invention has an injectivity which is more than oneand a half times higher than that of PUSHER 700 polymer. Further, theperformance of the polymer of this invention has much higher efficacy inthe matrix (distant from the wellbore region) than PUSHER 700 polymer,one of the leading commercial polymers.

In addition to their superior injectivity and mobility controlproperties, the partially hydrolyzed polyacrylamides of this inventionalso have a brine tolerance greater than that of conventionalpolyacrylamides such as PUSHER 700 polymer. The effect of salinity onthe matrix reciprocal relative mobility of a polymer of the presentinvention and PUSHER 700 polymer is graphically portrayed in FIG. 8 ofthe drawings. The polymer of this invention shows a surprisingly andconsistently higher tolerance for brine at all concentrations used.

To further establish the marked differences in properties between thepolymers of this invention and commercial products such as PUSHER 700polymer, shear tests were carried out under comparable conditions. Sheardegradation, which manifests itself as a loss in screen factor and/orviscosity, of the polymer of the present invention was less thanone-third the degradation of PUSHER 700 polymer.

A further significant property of the polymers of this invention overconventional partially hydrolyzed polyacrylamides is their excellentstability. Thus, by way of illustration, a polymer preparedapproximately eight months prior to testing was stored, withoutpreservatives, at approximately one percent concentration, withoutexcluding dissolved oxygen, and exhibited no apparent degradation.

Certain modifications of the process and system of this invention willoccur to those skilled in the art from a reading of the foregoingdescription. It should be understood that such description has beengiven by way of illustration and example and not by way of limitation.It is intended to include within the invention any such modifications asfall within the scope of the claims.

What is claimed is:
 1. A process for recovering oil from a subterraneanoil-bearing reservoir having located thereat at least one input well andat least one output well through which the oil in the reservoir isrecovered in response to the force applied by a displacement fluid;comprising: predetermining the injectivity and mobility propertiesrequired of an oil displacement emulsion and a mobility buffer to meetthe performance demands of a subterranean oil bearing reservoir both atan input well and in the matrix of the reservoir; providing an oildisplacement emulsion and a mobility buffer having the requiredpredetermined injectivity and mobility properties; injecting theemulsion into a reservoir through at least one input well formed in thereservoir; injecting into said input well the mobility buffer, saidbuffer being in the form of an aqueous solution of a polymer having anaverage molecular weight of about 10 million, and a molecular weightdistribution of about 0.02 to about 0.22 as determined by the ratio σ/Mwherein σ is the standard deviation of molecular weight of abundance ofa molecular species present in the polymer and M is the averagemolecular weight of the polymer, the reciprocal relative mobility ofsaid buffer being such that it is lower at and adjacent to the inputwell then it is in the matrix of the reservoir; and recovering oil at anoutput well formed in the reservoir.
 2. A process according to claim 1wherein the polymer is a partially hydrolyzed polyacrylamide.
 3. Aprocess accroding to claim 1 wherein the emulsion is a micellardispersion.
 4. A process according to claim 1 wherein the averagemolecular weight, the reciprocal relative mobility and/or the molecularweight distribution of the polymer in the leading or emulsion contactingportion of the aqueous polymer solution is greater than thecorresponding parameters of the polymer in the trailing or rearwardportion of the solution.
 5. A process according to claim 1 wherein atleast one additional mobility buffer in the form of an aqueous solutionof a polymer, and having injectivity and mobility propertiescorresponding to those of the first injected mobility buffer, isinjected into the input well after said first injected mobililty buffer,said at least one additional mobility buffer having an average molecularweight and a mobility such that the leading portion thereof will beessentially the same as the trailing portion of the first-mentionedmobility buffer.
 6. A process according to claim 1 wherein the polymerused in forming the mobility buffer is produced at the site of its use,and the average molecular weight and molecular weight distribution ofthe polymer are selectively regulated by altering the polymerizationparameters to provide in an essentially continuous operation an aqueouspolymer solution capable of meeting the performance demands of theformation.
 7. In a process for recovering oil from a subterraneanoil-bearing formation having located thereat at least one input well andat least one output well through which the oil in the formation isrecovered in response to the force applied by a displacement fluid,comprising: predetermining the injectivity and mobility propertiesrequired of an oil displacement fluid, in the form of an aqueous polymersolution, to meet the performance demands of an oil-bearing formationboth at an input well and in the matrix of the formation providing anaqueous polymer solution having the required predetermined injectivityand mobililty properties, said aqueous polymer solution beingcharacterized in that the polymer dissolved therein has an averagemolecular weight of about 2 million to about 10 million, and a molecularweight distribution of about 0.02 to about 0.22 as determined by theratio σ/M wherein σ is the standard deviation of molecular weight ofabundance of a molecular species present in the polymer and M is theaverage molecular weight of the polymer, the reciprocal relativemobility of the aqueous polymer being such that it is lower at andadjacent to the input well of the formation; injecting into an inputwell of the formation a polymer solution having the requiredpredetermined injectivity and mobility properties; injecting water fordriving the polymer solution through the formation; and recovering oilat an output well formed in the oil-bearing formation.
 8. In a processaccording to claim 7 wherein the polymer is a partially hydrolyzedpolyacrylamide.
 9. In a process according to claim 8 wherein from about20 to about 40 percent of the amide groups of the polymer arehydrolyzed.